Carrier Aggregation will be used in 3GPP LTE networks to provide improved data rates to users, which are also referred to as a mobile station or as user equipment (UE). Carrier aggregation consists of transmitting data to or receiving data from a UE on multiple carrier frequencies (“component carriers”). The wider bandwidth enables higher data rates. The present disclosure considers the problem of change of the primary cell of a mobile station. For carrier aggregation (CA) operation, a UE can be configured with a set of serving cells. Each of the serving cells has a downlink frequency and may also have an uplink frequency. The downlink frequencies of the different serving cells of a UE are distinct and the uplink frequencies of a UE are distinct. In the case of a frequency division duplex (FDD) system, the downlink and uplink frequencies of a serving cell are distinct. In the case of a time division duplex (TDD) system, the downlink and uplink frequencies of a serving cell are the same. Carrier aggregation operation can also be described in terms of “component carriers” or “serving frequencies”. The mobile station has a set of “downlink component carriers” or “downlink serving frequencies” (corresponding to the downlink frequencies of the serving cells) and “uplink component carriers” or “uplink serving frequencies” (corresponding to the uplink frequencies of the serving cells). In general, all the serving cells of a mobile station belong to the same base station (that is, all the downlink frequencies are transmitted from the same base station). However, there can be exceptions to this rule. For example, in a base station can transmit one or more downlink serving frequencies, while the remaining downlink serving frequencies are transmitted are transmitted by one or more remote base station equipments.
One of the serving cells is designated as a “primary cell”. Correspondingly there are “downlink primary frequency” and “downlink primary component carrier” which refer to the downlink frequency of the primary cell, and “uplink primary frequency” and “uplink primary component carrier” which refer to the uplink frequency of the primary cell. A serving cell that is not a primary cell is referred to as a secondary cell.
The communication link between the mobile station and the primary cell is a primary link in the communication between the mobile station and the network. Specifically, at the time of connection establishment between the mobile station and the network, a bi-directional communication link between the mobile station and the primary cell is established before any communication link is established between the mobile station and a secondary cell.
Carrier Aggregation procedures are designed such that the mobile station transmits the uplink control channel only to the primary cell (i.e., only on the uplink primary frequency). The uplink control channel carries various control information such as ACK/NACK indicators for downlink transmission from all serving cells, Channel Quality Indicator (CQI) information for all serving cells and scheduling requests (SR). Thus any interruption of uplink transmissions from the mobile station to the primary cell effectively results in interruption of almost all downlink communication (including all user data communication) through all of the serving cells.
If the communication link between the mobile station and the primary cell is interrupted, the communication between the mobile station and the network fails even if the communication link between the mobile station and a secondary cell is sustained. The primary cell provides security and encryption related information to the mobile station. Furthermore, the mobile station performs radio link monitoring of the primary cell but not of the secondary cells. If the radio link monitoring of the primary cell indicates that there is a radio link problem, the mobile station suspends all reception and transmission of signals. If there is no recovery from the radio link problem, the mobile station declares a radio link failure, breaks the connection to the serving cells and attempts to re-establish the connection to the network through a suitable cell. Such actions can result in significant interruptions of the communication link and also in irrecoverable failure of the communication link.
Given the importance of the link between the mobile station and the primary cell, in order to minimize problems associated with radio link quality, it is necessary to ensure that the primary cell is the best cell of the serving cells. Such a requirement then makes it necessary to change the primary cell when the primary cell is no longer the best cell. Change of the primary cell can be achieved by emulating a handover procedure, wherein the source cell is the current primary cell and the target cell is the new primary cell. However, such a procedure has significant drawbacks. The handover procedure is designed for mobility—that is, for transferring a mobile station from a cell at a first base station (source cell) to a cell at a second base station (target cell). Thus, the mobile station has to perform synchronization to the target cell before communication via the target cell can resume. In 3GPP LTE, the synchronization is performed by a random access procedure, i.e., transmitting on a random access channel and receiving a random access response. The synchronization procedure results in a significant interruption. Since the interruption includes an interruption of uplink transmission to the primary cell, it results in interruption or suspension of all downlink data between the network and the mobile station (as described above). Furthermore, the various communication protocols (such as medium access control and radio link control) and in the mobile station have to be reset and re-established before communication can resume through the target cell. The security key is also changed at the handover. The re-establishment of the protocols and the change of the security key can result in loss of any buffered packets at both the base station and the mobile station.
Therefore it is beneficial to have procedures for change of primary cell that remedy the deficiencies outlined above.